With the improved technologies on moving image data compression and hardware devices, recently commercial equipment for handling digital moving images has been increasing in number. Among such equipment, there is one type having a large capacity hard disk (HDD) for storing a large amount of moving image data, enabling to extend recording time. Another type provides a flash memory as a storage medium having a capacity of several tens of megabytes, mainly aiming at miniaturization.
In the former type, it is difficult to miniaturize the equipment because of the HDD size, while in the latter type there is a disadvantage of limited image recording time. When extending the recording time is intended in the latter type, it is necessary to raise compression rate at the cost of image quality. Therefore, each of the types has the own merit and demerit.
In order to satisfy both requirements of miniaturization and elongation of recording time, there is one solution that the recorded images be stored not into a recording device included in the equipment itself, but into a recording device provided outside the equipment being connected with communication means. Provision of recording device outside the equipment (or ‘terminal’) facilitates to miniaturize the camera terminal, and at the same time to obtain extended recording time by storing into a recording device having a sufficiently large capacity.
When using a less reliable network with relatively frequent bit errors such as a wireless network, packet transmission technology can be applied to achieve accurate data transmission. The data are packetized and arrival of the packet data is monitored at both communication ends. When a packet is not received, retransmission processing is performed. Using such a method, it is required to provide a means for guaranteeing data identity between the data to be transmitted and the data actually received on the other side.
A known technology to enable the above means is ARQ (automatic repeat request). Data are divided into frames to transmit, and retransmission request is issued when a received frame includes an error, thus an error free scheme is realized.
However, in the ARQ retransmission processing, there is a problem that when packet loss occurs frequently, the packets triggered by the retransmission processing are retransmitted frequently through the network, which produces undesirable influence to the transmission bandwidth for normal data packets. In some cases network congestion may occur, resulting in stoppage of normal data transmission. Therefore, the ARQ method is regarded as not a proper means for real-time data transmission.
In particular, digital moving image data created by a digital video camera are generated in real time and continuously. Accordingly, to say nothing of the extreme case of network congestion, when real-time data transmission is impeded, data being unable to transmit must be kept in the camera terminal. Such communication bottleneck may cause stoppage of further recording.
To the contrary, if a priority is given to guaranteeing real-time data transmission at the cost of data accuracy, one method is to suppress retransmission of lost packets so as to circumvent the communication problem. However, when using this method, it is not possible to regenerate accurate moving image using data stored in the network server because of the inaccurate transmission and storage. Thus complete recording function of moving images, which is a basic requirement to a digital video camera, cannot be achieved.